1. Field of the Invention
The present invention relates to a method for producing a novolak-type epoxy resin used for a encapsulating material and the like in the electric and electronic industries.
2. Description of the Related Art
In normal production of an o-cresol novolak-type epoxy resin, first, o-cresol is reacted with formalin to obtain an o-cresol novolak resin as an intermediate material. Then, the o-cresol novolak resin is fed into an epoxidation reactor together with epichlorohydrin and, if necessary, a solvent also, to allow for epoxidation. One of major factors for determining the properties or quality of the thus-produced epoxy resin is the molecular structure, in particular, skeleton structure of the novolak resin.
Whether or not a novolak resin has a desired molecular structure is normally determined by measuring the softening point of the resin. In other words, a novolak resin having a specific softening point is used for epoxidation so that an epoxy resin having a desired molecular structure or desired properties such as softening point, etc., can be produced.
Therefore, in order to produce an epoxy resin suitable for a desired usage, it is required to prepare a novolak resin having a predetermined softening point.
In a conventional production process of a novolak resin, a novolak resin produced in a reactor is once solidified by cooling into flakes to avoid possible quality degradation and the like. The resin flakes are stored in an appropriate container so that a required amount of the resin can be fed into a reactor when an epoxy resin is to be produced.
The above process has following problems. The operations of putting the novolak resin in a container and taking a predetermined amount of the resin out from the container to feed it into a reactor are mostly performed manually. Dust of the novolak resin generated during these operations may worsen the working environment.
Another problem is that the resultant epoxy resin is colored. In particular, when the novolak resin is stored at higher temperature than normal temperature as in summer, it may degrade by oxidation, causing quality degradation of the resultant epoxy resin such as significant coloring, an increase in epoxy equivalent weight, and a change in softening point.
The inventors have intensively studied for solving the above problems and found that the novolak resin can be prevented from quality degradation by storing it in a molten state in an inert gas atmosphere, and that this storage in a molten state allows for in-line handling of the resin, improving operability and the working environment.